Dense films can be obtained from solutions of poly-.alpha.,.beta.,.beta.-trifluorostyrene. However, the brittleness of these films greatly limits their application. Films obtained from some sulfonated poly-.alpha.,.beta.,.beta.-trifluorostyrenes can be used as ion-exchange membranes. However, such films often have unfavorable mechanical properties when wet, and are known to be very brittle in the dry state (see, for example, Russian Chemical Reviews, Vol. 59, p. 583 (1988)). Such films are of little practical use in fuel cells due to their poor physical properties. Some improvements in mechanical properties have been achieved by blending sulfonated poly-.alpha.,.beta.,.beta.-trifluorostyrene with polyvinylidene fluoride and triethyl phosphate plasticizer, but these films remained unsatisfactory for application in electrochemical cells (see Fuel Cell Handbook, A. J. Appleby, published by Van Nostrand Reinhold, p. 286 (1989)).
U.S. Pat. No. 5,422,411 and the related patent applications mentioned above describe various polymeric compositions incorporating substituted .alpha.,.beta.,.beta.-trifluorostyrenes and some cases further incorporating substituted ethylenes. Typically these compositions, as membranes, possess favorable mechanical properties compared to poly-.alpha.,.beta.,.beta.-trifluorostyrene and sulfonated poly-.alpha.,.beta.,.beta.-trifluorostyrene, although some of the membranes have a tendency to become brittle in the fully dehydrated state, depending, for example, on the equivalent weight. This effect is most apparent at equivalent weights below approximately 380 g/mol. Ion-exchange membranes derived from these polymeric compositions are suitable for many applications, including use in electrochemical applications, such as fuel cells.
For ease of handling, for example, in the preparation of membrane electrode assemblies for use in electrochemical fuel cells, the mechanical strength of the membrane in the dry state is important. In electrochemical applications, such as electrolytic cells and fuel cells, the dimensional stability (changes in the dimensions of the membrane due to changes in the degree of hydration) of the membrane during operation is also important. However, to improve performance, it is generally desirable to reduce membrane thickness and to decrease the equivalent weight (thereby increasing the water content) of the membrane electrolyte, both of which tend to decrease both the mechanical strength in the dry state and the dimensional stability in the wet state. One way to improve mechanical strength and dimensional stability in ionomeric membranes is through use of a substrate or support material, to give a composite membrane. The substrate is selected so that it imparts mechanical strength and dimensional stability to the membrane. The substrate material can be combined with the membrane polymeric material to form a composite membrane in a variety of ways. For example, if possible, an unsupported membrane can be preformed and then laminated to the porous substrate. Alternatively, a solution of the polymer can be impregnated into the porous substrate material, and the composite membrane subsequently dried. The formation of composite membranes via impregnation provides a more intimate contact between the two components, thus giving advantages over standard lamination approaches.
Composite ion-exchange membranes prepared by impregnating commercially available porous polytetrafluoroethylene film (Gore-tex.RTM.) with Nafion.RTM., a perfluorosulfonate ionomer, have been described in Journal of the Electrochemical Society, Vol. 132, pp. 514-515 (1985). The major goal in the study was to develop a composite membrane with the desirable chemical and mechanical features of Nafion.RTM., but which could be produced at low cost. Indeed, based on the polymer loadings necessary to produce these composite membranes, they are a low cost alternative to the costly perfluorosulfonic acid membranes. As indicated above, however, these perfluorosulfonate ionomers are known to form membranes suitable for use in electrochemical applications without the use of a substrate.
It has been discovered that polymers which have a tendency to become brittle in the dehydrated state can be rendered mechanically stable, even in the fully dehydrated state, by impregnation into suitable substrates.
Furthermore, it has been discovered that even polymers which are poor film formers, or polymers which form films with mechanical properties and dimensional stability which would preclude their use in electrochemical and other applications, can be made into composite membranes through incorporation into a suitable substrate. The resulting composite membranes have the desired physical properties for use in a wide range of applications.